1. Field of the Invention
This invention is directed to ion exchange compositions; and more particularly to a titania bound ion exchange composition comprising a crystalline sodium titanate and a hydrolyzable titanium compound.
2. Description of the Art
Ever since nuclear weapons were first produced at the end of World War II, large amounts of nuclear waste have been generated and stored at various facilities. The nuclear waste, which consists largely of the byproducts of uranium and plutonium production and purification, was disposed of in ways which were deemed suitable at the time, but which in retrospect are now inadequate. Much of the nuclear waste is now stored in tanks as a highly alkaline mixture of salts and liquids which, if not recovered and properly remediated, will potentially create severe environmental problems.
Most of the stored aqueous nuclear waste is alkaline (pH 14), and contains high concentrations of sodium nitrate. The tanks contain various complexing agents, fission products, transuranic elements and other materials. Much of the stored nuclear waste is in the form of sludge created when alkali was added to the waste to prevent tank corrosion. Some of the radioactive material has been incorporated into salt cakes which is the evaporative product of the alkaline aqueous material. It is desired to remove the radioactive elements from the waste in order to allow for subsequent safe disposal of the non-radioactive materials. The removal of two of the metallic radionuclides, cesium and strontium, is particularly important because their half-lives are long enough to represent a hazard for an extended period of time.
Sodium nonatitanate is known as a strontium ion exchanger. J. Lehto; J. Radioanal. Nucl. Chem. Letters, 118: 1-13 (1987) describes the ion exchange behavior of Na.sub.4 Ti.sub.9 O.sub.20.xH.sub.2 O toward strontium. The sodium titanate was prepared hydrothermally at 300.degree. C. followed by boiling with NaOH.
Other references that disclose the preparation and use of sodium titanates as strontium ion exchangers include R. G. Dosch, "Final Report on the Application of Titanates, Niobates, and Tantalates to Neutralized Defense Waste Contamination Materials Properties, Physical Forms and Regeneration Techniques; National Technical Information Service" (1981); R. M. Merrill; Journal of Radioanalytical Chemistry; 43: 93-100 (1978); J. Lehto et al.; J. Chem. Soc. Dalton Trans., 101-103 (1989); and S. P. Mishra et al.: J. Radioanalytical and Nuclear Chemistry, Articles; 162:2, 299-305 (1992).
Various types of sodium titanates are described in the prior art as well as various uses for sodium titanates as an ion exchanger for ions besides strontium. M. Watanabe; Journal of Solid State Chemistry, 36: 91-96 (1981) describes the preparation of sodium titanate compounds from hydrothermal reactions involving TiO.sub.2 with NaOH. The titanates described were TiO.sub.2, Na.sub.2 OnTi.sub.2 Na.sub.x TiO.sub.2. J. Akimoto et al., Journal of Solid State Chemistry, 90: 147-154 (1991) describes the synthesis of monosodium titanates NaTi.sub.8 O.sub.13, which are distinct from the sodium nonatitanates disclosed here. H. Leinonen et al., Reactive Polymers, 23: 221-228 (1994) describes the use of sodium titanates as ion exchangers for nickel and zinc.
Lehto et al.; Radiochem. Radioanal. Letters, 50:6, 375-384 (1982) describes the effects of gamma radiation on sodium titanate and other solid ion exchangers. The study concluded that gamma radiation had very little impact on the strontium ion exchange capacity of sodium titanate.
PCT Application WO 94/19277 discloses silico-titanates and methods for making and using them. The silico-titanates disclosed are useful for removing cesium from radioactive wastes. U.S. Pat. No. 4,156,646 discloses removal of plutonium and americium from aqueous alkaline waste solutions using sodium titanate ion exchangers. The sodium titanate used is a monosodium titanate. U.S. Pat, No. 5,352,644 describes a titania bound zeolite made by combining the zeolite, a low acidity titania binder material, and an aqueous slurry of titanium oxide hydrate.